The tubes, tube sheets and firebox sheets are subject to a torturous thermal and chemical environment. Coal and oil both produce sulfuric acid which is highly corrosive, along with thousands of other chemicals that eat away at the metal. There are also physical particles such as soot, dirt, and sand that scour away material. They are also subject turbulent corrosive action on the water side. Alkaline water produces scale witch will cause hot spots and help eat away at the stays and sheets. Other minerals and solids in solution also waste away material.
Thermal considerations such fire intensity and water level and circulation also play hell with the material. The outer wrapper sheets are typically thicker than internal sheets because they have to support the entire structure and the immense strain of being under intense pressure.
As for design, there are thousands of different boiler designs, each with specific purposes. But I think you are asking about locomotive boilers in specific. Here are the basics, riveted joints are the weak points, specifically in the cylindrical and conical sections. The weakest and oldest design is a single set lap seam. Next is the butt strap joint , and the double butt strap joint which refers to the number of rivets and the shear forces. A lap seam is in single shear, with torsion while a butt joint has the force applied twice across a rivet with uniform lateral force (double shear). A double butt strap has a larger internal strap that has addition rows of stays supporting the structure. This is the most modern and strongest design.
Boilers are also stayed differently, crown bar which is an old design generally found on wagon top boilers. Simple radial stays which are fixed and more modern designs have flexible stays which are fitted like a ball and socket with a steam tight cap. Each design has its strengths and weaknesses.
Take a look at this website, might help explain boiler structure.
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steamtraction.farmcollector.com]
